Induction charging



April 26, 1960 L. E. WALKUP 3 v INDUCTION CHARGING Filed Jan. 9, 1957 2 Sheets-Sheet 1 TRANSFER DEVELOPMENT POWER 32 SUPPLY.

H \G H VOLTAGE SOURCE.

0c 52 POWER 55 SUPPLY 5g 53% Fi 1 INVENTOR.

LEWIS E.WALKUP April 26, 1960 Filed Jan. 9, 1957 t mns er L. E. WALKUP 2,934,649

INDUCTION CHARGING 2 Sheets-Sheet 2 "58025 50554045 50556065 AIR GAP, MICRONS NOHQHN 213d SIIOA 8V9 alv EIHL NI 0131; DRLLDEHH INVENTOR. LEWIS E. WALKUP ATM United States Patent INDUCTION CHARGING Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc.

Application January 9, 1957, Serial No. 633,331 13 Claims. or. 250-495 paratus for electrophotography or xerography wherein an electrostatic charge applied to the surface of a photoconductive insulating layer is selectively dissipated by exposure to a pattern of light and shadow to be recorded, and the result is an electrostatic latent image on the surface of thephotoconductive insulating layer corresponding to the pattern of light and shadow to which the photoconductive insulating layer was exposed. conventionally in the art now known as xerography, an electrostatic image may be formed in this manner and may be utilized as desired, for example, by development or deposition of finely divided material in conformity with the charge pattern, optionally together with transfer of the developed image to a print-receiving surface.

Now, in accordance with the present invention, there are provided novel means, methods, and apparatus for the application of electrostatic charge to the surface of an insulating layer and the primary application of the present invention is sensitizing the xerographic plate through the deposition of uniform electrostatic charge on its surface.

The general nature of the invention having been set forth, other objects and advantages will become apparent and obvious from the detailed discussion of the invention which follows, and the invention will now be described illustratively in terms of the following specification and drawing, in which:

Figure 1 is an isometric view of an embodiment of apparatus for charging a photoconductive insulating body according to this invention;

Figure 2 includes other xerographic operations or stations and a further embodiment of apparatus for charging a photoconductive insulating body in accordance with this invention;

Figure 3 is a fragmentary view of another embodiment of a charging apparatus for example, for a device of the type illustrated in Figure 2; and,

Figure 4 is a set of curves defining charge transfer in accordance with air ionization theory generally believed to be applicable to this invention.

Referring now to Figure 1 wherein there is illustrated an embodiment of this invention, the plate comprising photoconductive insulating layer 11 overlying a conductive backing member 12 is positioned in substantially parallel relationship or face-to-face relationship with elec trode 26 made up of conductive material and carrying an insulating coating 24 on its surface. A handle 27 of tive photoconductive insulating layer.

electrically insulating material is attached to electrode .26

or, optionally, the handle may comprise any material and in such event would be electrically separated from electrode 26 by an insulating section. Electrode 26 is connected through lead line 28 and switch 30 to power supply 31. Lead line 32 connects power supply 31 to backing member 12 of plate 10.

In order to charge the surface of the photoconductive insulating layer 11 the insulating coating or layer 24 is first positioned in face-to-face relationship with the surface of photo-conductive insulating layer 11. Switch 30 is closed either during the approach of electrode 26 to the surface of the photoconductive insulating layer 11 .or while the insulating surfaces 24 and 11 are in face-to-face relationshipand preferably switch 30 remains closed (potential being applied) during separation of electrode 26 from the surface of photoconductive insulating layer 11. When the conditions of charge migration as will be described more fully hereinafter are met the surface of layer 11 will be in a charged condition following removal of electrode 26 away from the surface of photoconductive insulating layer 11.

Power supply 31 preferably is of a reversible variety, i.e., able to supply positive or negative potential as desired and is preferably variable from O to many thousand volts DC. potential. This type of power supply allows charging of the surface of photoconductive insulating layer 11 either positively or negatively as is desired. For example, if backing member 12 is placed at a ground potential and electrode 26 is raised to a positive potential and a sufficient potential is supplied to bring about charge migration between the facing surfaces, the surface of photoconductive insulating layer 11 will be charged positively; whereas, if backing member 12 of plate 10 is placed, for example, at ground potential and electrode 26 is placed at a raised negative potential, the surface of photoconductive insulating layer 11 will be raised to a negative electrostatic charge potential when the conditions are such as to permit charge migration.

Reference is now had to Figure 2 wherein an automatic type xerographic machine is illustrated. The plate 10 comprising a photoconductive insulating layer 11 and a conductive backing member 12 is in this embodiment in the shape of a rotary drum. Positioned along the circum fcrence of the drum are various xerographic process stations. These various stations include the charging station generally designated 33, exposure station generally designated 35, a development station designated 36, a transfer station 37, and a cleaning station 38. The drum 10 is driven by motor 40 in the direction indicated through belt drive 41 connected at axle 42 of drum 10. Thus, the plate is carried first to the charging station 33 whereat an electrostatic charge is placed on the surface of photoconductive insulating layer 11, thereby sensitizing the plate. The plate is then carried to exposure station 35 where an image of light and shadow is projected to the surface of photoconductive insulating layer 11 and is converted into an electric charge pattern by selectively dissipating electrostatic charge in areas where light or other activating radiation strikes the surface of the sensi- Next, the drum is carried to development station 36 whereat particles carrying electrostatic charge are contacted against the surface of the photoconductive insulating layer 11 and deposit in accordance with the electrostatic field of force which exists between the charges on the surface of photoated at development station 36 is transferred to a sheet or web of material 7 fed from supply spool 8 to takeup roll 9 and optionally fixed thereon. Following transfer candescent lamp or fluorescent lamp or through the use of other techniques generally known to the art.

Charging station 33 in this embodiment includes conductive drum 45 positioned to present a moebius strip endless belt 46 of insulating material into contiguous relationship or in virtual contact with the surface of photoconductive insulating layer 11. Drum 45 is driven by motor 47 through drive belt 48 and the drum rotates in the direction indicated to move endless belt 46 at a linear speed equal to the linear speed of photoconductive insulating layer 11 of drum 10. Endless belt 46 also passes over freely rotating wheel 50. Power supply 31 as in the embodiment described in connection with Figure 1 is adapted to supply either positive or negative potential and is connected through lead wire 32 to backing member 12 of drum 19 and through lead wire 28 to conductive drum 45.

As in the case of Figure 1, if the conditions of charge migration through the gap are met as will be discussed more fully hereinafter, a potential applied between conductive member 45 and backing member 12 will cause charge deposition on the surface of the photoconductive insulating layer 11. In this embodiment when insulating Web 46 is between conductive member 45 and drum and a potential is applied, the surface of the insulator attains an intermediate potential between the potential placed on member 45 and the potential placed on backing member 12, and an electric field exists between the facing insulating surfaces. When the electric field is intense enough to create charge migration between the facing surfaces whether it be as the insulating endless belt 46 is moving into face-to-face relationship with the photoconductive insulator 11, while it is in face-to-face relationship with the photoconductive insulator 11, or while it is moving out of face-toface relationship, whether face-to-face relationship is contact or includes a minute air gap, a charge is placed on the photoconductive insulator 11. In addition, however, as will appear more fully hereinafter, charge is either deposited or removed from the surface of the insulating web 46. In order to maintain consistency and uniformity in the amount of charge deposited, the web surface must be made neutral prior to its return to the charging position so that it can again attain the same intermediate potential for charging purposes. This is accomplished in this embodiment through the use of a single surfaced belt. When using such a belt a point on its surface which is outward as it passes between drum 10 and member 45 will be inward during its next pass therethrough. While outward it will become charged and while inward any charge thereon will be removed through conductive member 45. Thus while outward during the next pass the surface will be electrically neutral as it comes into the charging area. To assure that the surface be neutralized it is generally preferred that member 45 be maintained at ground potential. As in the previous embodiment separation is also accomplished in this embodiment with the external potential applied. Generally, also, the use of a moebius strip for charging purposes is particularly valuable in connection with plates having a narrow width since the strip becomes increasingly more difiicult to handle as its width increases.

Reference is now had to Figure 3 wherein another charging mechanism is illustrated. As in the previous embodiment power supply 31 connected through leads 32 and 28 applies potential across conductive core 51 of roller 52 covered on its outer surface with a uniform layer of insulating material 53 and conductive backing thereof of photoconductive insulating material 11. In this embodiment charging takes place as was described in the last embodiment. However, the charges deposited on or removed from the surface of insulating layer 53 of drum member 52 during charge migration are neutralized in this embodiment by corona discharge electrode.55 comprising an outer shield 56 which as is illustrated is grounded and corona discharge wires 57. Corona discharge wires 57 are supplied with a high voltage from high voltage source 58 and the corona discharge electrode is adjusted preferably to supply a neutralizing charge to the surface of insulator 53. It is possible, however, in this embodiment to precharge the surface of insulator 53 if desired. Other techniques of neutralizing the insulating surface may be used in connection with this embodiment or others which will readily occur to those skilled in the art as, for example, contacting the surface with a conductive roller or the like at a grounded or properly raised potential to result in neutralization.

Reference is now had to Figure 4. In this figure there appears a chart comprising a family of hyperbolas, each marked with a different voltage and also a curve designated critical stress. The family of curves show the electrical stress for particular air gap distances employing particular applied voltages. They also show how electric stress varies in the gap as the gap width varies for applied voltages.

Electric stress through a material as, for example, a uniformly thick layer of material, may be computed by dividing the voltage applied across the material by the quotient of the thickness of the material divided by the dielectric constant of the material. When dealing with air as the layer of material separating the electrodes, the dielectric constant of air may be taken as one and, accordingly, the field applied through the air is found by dividingthe voltage applied across the air by the thickness of the layer. Since in this invention narrow gap widths are involved which, for example, generally are in the order of less than microns, the stress curves are based on a micron spacing scale, and stress when read from this figure, therefore, is stated in terms of volts per micron. Further, since generally the experiments relating to this invention have been carried out in air, the dielectric constant of which may be taken as 1, and since generally it is simpler to perform this invention in air as distinguished from gases and the like which might, for example, require closed-off areas, electric stress in the family of curves of Figure 4 represents electric stress for the particular applied voltages across air gap distances. in Figure 4 have been computed by applying a particular potential between two electrodes separated by an air gap and varying the air gap while maintaining the potential constant, it should be apparent that stress curves may be drawn taking into account any particular material, that is, in this case any fluid (gas or liquid), across which a voltage is applied. By using various potentials, data to draw the family of stress curves of Figure 4 are obtained and the inclusion of use of other fluids is intended herein.

The critical stress curve defines electric field strength in a gap which will sustain air breakdown. It is based on breakdown of air at atmospheric pressure since the family of hyperbolas are based on field strength in an air gap at atmospheric pressure. However, it is to be realized that critical stress curves for other materials and pressures may be computed if, as a matter of fact, other dielectric materials or air under pressure comprise the gap material or, as will be illustrated more fully hereinafter, the curves of this figure may be used when the other materials in the system are converted into equivalent air thickness of equivalent air gaps.

When one is dealing with two surfaces which define a gap and which carry the applied potential, the critical Although the family of curves found tea s stress curve shown in this figure is used; However, if.

additional material such as the photoconductive insulating layer of'a xerographic plate is positioned between the electrodes, one must take into account the. voltage drop across this additional material to determinevoltage applied across the gap for the particular gap distance involved. Similarly, where a further insulating layer is between the electrodes, as, for example, as is found in Figures 1, 2, and 3, the voltage drop across this material must be determined to find the voltage applied across the gap. The voltage across the solid dielectrics between the electrodes is found by considering the solid dielectrics and the gap as capacitors in series; for example, if the air equivalent thickness of the photoconductor is equal to the air gap then it could be expected that an equal amount of voltage would appear across the solid dielectric as appears acrossthe air gap. Realizing that each element between the electrode acts as a capacitor in Series with each other element between the electrodes, one can readily compute the voltage applied across the gap by deducting the voltage appearing across the solid dielectrics between the electrodes. Knowing the voltage across the gap one may readily use the curves found in Figure 4 to determine whether the applied voltage across the gap is sufficient to bring about critical stress and if the electrodes remain stationary relative to one another using the particular gap distance separating the solid dielectrics and the particular applied voltage across the gap, one can determine whether critical stress is maintained or exceeded for that particular arrangement.

Although only positive electric stress is plotted in this figure, there exists a similar body of negative electric stress curves and a negative critical stress curve. The negative curves are mirror images of those shown in this figure.

The chart of this figure is used to predict the outcome of charge transfer through gaps for various arrangements. For example, looking at the family of stress curves for different voltages through various air gaps, it becomes apparent that, if the electrodes to which the voltage is applied approach one another, the electric stress in the gap separating the electrodes increases. However, the factthat stress becomes high as electrodes approach does not assure the transfer of charge from one electrode to the other. To determine when charge will transfer, it is necessary to know when the stress will produce dielectric failure in the gap. This is found from the critical stress curve. Examination of the critical stress curve shows that if the electrodes come closer together more stress is needed to bring about dielectric failure and, in fact, with an air gap ranging from to about 2 microns, dielectric failure in the gap may not take place. Also, the chart of this figure shows that, with about two hundred volts applied between electrodes, at no time will the stress in the gap be above critical stress for the spacings involved and, thus, no breakdown and no transfer should take place. On the other hand, if about a thousand volts is applied to the electrodes then as is shown by the graph dielectric failure will readily take place.

In the interest of following through the application of information as supplied by Figure 4 and for better understanding of charge migration according to the embodiments in Figures 1, 2, and 3, it can be assumed that the photoconductive layer of the plate in Figures 1, 2, and 3 comprise a selenium layer having a dielectric constant of 6 and having a thickness of 48 microns and the insulating material illustrated in Figures 1, 2, and 3 has a dielectric constant of 3 and is 24 microns thick. In such a situation the air equivalent thickness of the selenium layer is equal to 8 microns (48 divided by 6) and the air equivalent thickness of the insulating layer is equal to 8 microns (24 microns divided by 3). If, further, in Figures 1, 2, and 3 there is a gap distance of 8 microns then it can be expected that the applied voltage will divide equally between the air gap and each of" the solid dielectrics in the gap. Thus, and? for ex ample, if in the area of charging of Figure 2 the potential applied between the backing member and the conductive roller is in the order of 1200 volts, 400 volts would ap pear across the photoconductive insulator and 400 volts would appear across the insulating belt and 400 volts would appear across the gap. With 400 volts applied across an 8 micron gap, Figure 4 shows that dielectric failure in the gap will take place. If 1800 volts were applied between these same electrodes then 600 volts would appear across the gap, and as is shown by Figure 4 dielectric failure in the gap would take place. If, on

the other hand, 900 volts were applied across thesesame electrodes, critical stress is not attained and dielectric failure would not take place since only 300 volts appear across the gap.

Another interesting point to consider is the happening in a system in which the applied potential for the gap involved is too little to bring about dielectric failure in the gap for charge transfer but is suflicient to bring about charge transfer for a larger gap width and separation of the electrode from the plate surface occurs while the potential is applied. For example, assuming a gap distance in the order of less than 2 microns then breakdown in the gap cannot occur and no charge will transfer. If, however, the external potential is maintained as the electrode is separated from the plate surface and a gap voltage is attained of more than 375 volts at a gap distance of approximately 8 /2 microns charge transfer will take place. Thus, if the applied potential continues to be applied, as is preferred, during separation, then even if charge did not transfer while the electrodes were in face-to-face relationship, charge can be expected to transfer as the electrodes are separated if the applied potential is above critical stress for some distance through which the electrodes travel. Similarly as charge can transfer during separation, charge can transfer as the electrodes come into face-toface relationship or are brought together, if, when they are brought together a potential is applied to the electrodes. Charge transfers across the gap when the gap distance and the applied potential brings the stress in the gap above critical stress.

It is further noted in connection with charge transfer that if the electrodes are uniformly at the same potential and if the electric field is uniform throughout the entire gap, then charge will transfer uniformly across the gap giving rise to uniform charge deposition on the surface defining the gap. One manner of assuring uniformity of charge deposition is to start with an electrically neutral insulating surface to be positioned against the plate surface.

When critical stress is attained ions which are normally present in the gap are accelerated into collisions with nearby air molecules thereby creating additional ions which similarly collide with molecules to create more ions, etc. Also, charges are released from the surfaces defining the gap by collisions with the surfaces by moving ions in the gap creating additional ions in the gap and the created ions traveling in the space between the surfaces deposit on the surfaces controlled by the electric field thereby producing the charged photoconductive layer. The ions created, it is to be realized, are both positive and negative and the positively charged ions move to the negative surface whereas the negatively charged ions move to the positive electrode resulting in neutralization of charges which exist on the respective electrodes and also resulting in the deposition of new charges raising the amount of charge deposited on the electrode surface. For example, where there exists negative charges on a surface and positive charges are moved to the surface neutralization takes place and the charge density of the negatively charged surface is reduced. If, on the other hand, the surface is substantially at a neutral potential and charges are moved to the surface by. an electric field in thegap, then deposition of additional or new charges takes place on the surface thereby raising the charge density as controlled by the field on the previously neutral surface. Thus, in a real sense air ionization in the gap creates a conductive gap and allows charge flow between the surfaces defining the gap resulting in charge migration or charge transfer between the surfaces. This type of air ionization and ion travel continues while the electric stress in the gap is above critical stress and for a slight stress below critical stress. Once current flow or ion movement in the gap has started and as deposition takes place the electric stress in the gap is reduced by the deposited charges until charge migration stops. For example, if stress across the gap is 300 volts above critical stress and if the gap is defined by two insulating surfaces, since charge is neither destroyed nor created within the gap area and charge does not leak out of the gap, an equivalent amount of charge deposits on each surface defining the gap as controlled by the electric field in the gap. Accordingly, where one is dealing with 300 volts above critical stress and, depending on the dielectrics defining the gap, charge can deposit on each insulating surface to vary each surface by 150 volts (one surface being varied upward and the other downward thereby dropping the stress in the gap beneath critical stress).

To date, although many materials which can be used as the photo-conductive layer or as the other insulating layer defining the gap have been investigated, not all possible materials have been examined. Thus, although experimental work tends to prove that the theory of gas ionization accounts primarily for the transfer of charges (and, for this reason, gas ionization has been discussed in great detail), field emission, secondary emission, and the like may also enter into operation in connection with this invention, depending on various factors. For this reason the term field discharge is used in the claims and the body of this invention, and is intended to mean that limited form of discharge within the scope of this invention which results in charge transfer through a gap through the application of intense electric fields through short fluid gaps, which fields, however, are not intense enough to create spark discharge or, because of the close proximity of the adjacent surfaces defining the gap, spark discharge in the usual sense is prevented due to the limitations placed on the paths of travel on the ions in this space. 1

When such other phenomena as field emission or the like come into operation, slight variations from the predictions which are possible under the air breakdown theory described in detail in this specification can be expected. However, it is noted that for many materials sumcient information is available to determine when field emission, for example, will take place, and if the field emission curve for the particular-material is positioned on the curves illustrated in Figure 4, knowing the potential applied allows one to determine whether field emission will come into play or whether, as a matter of fact, air ionization only accounts for charge transfer. For example, where transfer of charge results with a gap width of less than 2 microns, it can be expected that a theory other than air ionization accounts for transfer or that atmospheric pressure is not present or the like. If field emission alone or in combination with other known theories of operation account for transfer, new curves may be drawn for the particular system, thereby allow ing complete predictability for the system. It is noted further that other phenomena, not all presently known, also seem to cause slight variances from the air ionization theory. For example, it has been found that when amorphous selenium is used as the photoconductor, variations from theory result which are presently believed to be a form of clean-up process within the selenium layer in which carries are first swept from the selenium, thereby establishing conditions allowing charge to transfer in accordancewith theory. However, even though results in connection with this invention may vary slightly from the theoretical predictions which can readily be made following air breakdown theory, the air breakdown theory is extremely helpful in setting up predictable results which, as a practical matter, will be different from experimental results only by an amount which might otherwise be considered experimental error. Yet, this variation in results, it is to be realized, is due to the other phenomena which are included with air breakdown in the term field discharge, which also come into play during charge formation through charge transfer across a gap while intense electric fields are applied across a narrow gap.

Within the general scope of the invention as specifical ly illustrated in the foregoing example, it is to be realized that numerous variations and modifications can be made. Thus, for example, it is disclosed that the member being charged comprises a metal plate having a photoconductive insulating layer thereon. It is to be realized that the method of the present invention is adapted to the charging. of photoconductive bodies in darkness and, thus, charging of an insulator is taking place. As to photoconductors, any of those generally used in xerography including, for example, photoconductive insulating selenium, anthracene and the like, and photoconductive bodies containing photoconductors dispersed in binder films and other photoconductive members in general may be sensitized according to this invention. Also, the backing member of the photoconductive insulating layer may comprise a metallic surface, a conductive glass surface, a conductive plastic member, conductive webs or other surfaces able to conduct electricity particularly adapted for xerography. It is also to be realized that a photoconductor not backed by a conductor can also be sensitized according to this invention. Such member may be self-supporting, backed by an insulator or the like. In addition, although the electric field ha been illustrated as applied across electrodes, it is to be realized that electrostatic charging as with corona or the like may be employed to create the electric fields desired and such modification as well as other similar ones generally known to the art are intended to be included herein. It is preferred, however, to back the plate and insulator with a conductive member whether it be a layer, roller or the like.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. The method of placing a charge on a xerographic plate comprising, positioning an insulating surface in face-to-face relationship and in virtual contact with a surface of a photoconductive insulating layer of the xerographic plate, and applying an electrical field above critical stress between the facing surfaces to cause field discharge therebetween and charge deposition across the insulating surface of the xerographic plate.

2. The method of placing a charge on a xerographic plate comprising, positioning an insulating surface in face-to-face relationship and in virtual contact with a surface of a photoconductive insulating layer of the xerographic plate, applying an electric potential between the facing surfaces, and while the potential continues to be applied separating the insulating surface from the surface of the photoconductive insulating layer, said applied potential being above critical stress between the facing surfaces to cause field discharge therebetween and charge deposition across the insulating surface of the xerographic plate at least prior to complete separation.

3. The method of placing a charge on a xerographic plate comprising a photoconductive insulating layer over.-

lying a conductive backing member, said method comprising positioning adjacent to and in virtual contact with the photoconductive insulating layer and across the area to be sensitized an electrically neutral insulating layer backed by a conductive layer, and applying a potential difference across the conductive backing member of the plate and the conductive layer of the insulator of a sufficient intensity to create a field above critical stress between the surface of the insulator and the surface of the photoconductive insulating layer resulting in air breakdown between the surface of the insulator and the surface of the photoconductive insulating layer thereby electrostatically charging the photoconductive insulating layer and reducing the stress between the surface of the insulator and the photoconductive insulating layer to a level of about critical stress.

4. The method of placing a charge on a Xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member, said method comprising positioning adjacent to and in virtual contact with the photoconductive insulating layer and across the area to be sensitized an electrically neutral insulating layer backed by a conductive layer, applying a potential difference across the conductive backing member of the plate and the conductive layer of the insulator, said potential difference being of a sufficient intensity to cause, in the absence of conduction through said photoconductive insulating layer, field discharge between the surface of the insulator and the surface of the photoconductive insulating layer, cutting off the applied potential, and separating the insulating layer from the surface of the plate.

5. The method of placing a charge on a Xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member, said method comprising positioning adjacent to and spaced apart from the photoconductive insulating layer by a substantially uniform and minute fluid gap an electrically neutral insulating layer backed by a conductive layer, and applying a potential difference across the conductive backing member of the plate and the conductive layer of the insulator of a sufficient intensity for the particular gap separating the surfaces to cause field discharge between the surfaces and sensitization of the plate,

6. The method of placing a charge on a Xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member, said method comprising positioning adjacent to and in virtual contact with the photoconductive insulating layer and across the area to be sensitized an insulating layer backed by a conductive layer, applying a potential difference across the conductive backing member of the plate and conductive layer of the insulator, and while the applied potential difference is maintained separating the insulating layer and its conductive layer from the plate, said applied potential difference being of a sufficient intensity to cause field discharge between the surface of the insulator and the surface of the photoconductive insulating layer at least prior to complete separation of the insulating layer from the photoconductive insulating layer thereby resulting in charge deposition across the surface of the Xerographic plate and plate sensitization.

7. Xerographic charging apparatus comprising means to present an electrically neutral insulating surface to the surface of a photoconductive insulating layer of a Xerographic plate, means to apply an electric potential difference to create an electric field above critical stress between such surfaces, and means to separate said surfaces while the potential difference remains applied to cause charge migration due to field discharge between said surfaces.

8. Xerographic charging apparatus comprising means to present an electrically neutral insulating surface to the surface of a photoconductive insulating layer of a xerographic plate, means to apply an electric potential difference between said surfaces above critical stress, means to separate said surfaces while the potential dilference remains applied to cause field discharge between said surfaces, and means to electrically neutralize said insulating surface prior to bringing it in contact again with the surface of a photoconductive insulating layer of a Xerographic plate.

9. A method of placing a charge on a Xerographic plate comprising positioning an electrode in face-to-face relationship and in virtual contact with the surface of a photoconductive insulating layer of a Xerographic plate, said electrode being continuous and substantially planar over the area to be charged, applying to said electrode an electrical potential in respect to said plate, and while the potential continues to be applied removing the electrode from the plate, said applied potential being of sufficient intensity to create a field above critical stress between the electrode and the plate surface at least prior to complete separation of the electrode from the plate surface to cause charging through field discharge,

10. In a method of charging a Xerographic plate wherein charge is placed on an insulating surface and is transferred from said surface to the Xerographic plate in the absence of charge flow through said plate, the steps comprising positioning a charged insulating surface in virtual contact and in face-to-face relationship with a Xerographic plate and applying an intense electric field above critical stress between said surface and the xerographic plate to cause charge, created through field discharge, to migrate from the insulating surface to the Xerographic plate.

11. Xerographic charging apparatus in accordance with claim 8 in which said means to present an electrically neutral insulating surface comprises an insulating surface on a roller.

12. Apparatus in accordance with claim 11 in which said means to neutralize the insulating surface comprises a corona discharge electrode positioned to direct ions to said insulating surface on said roller.

13. Apparatus in accordance with claim 8 in which said insulating surface comprises a surface of a moebius strip.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,558,900 Hooper July 3, 1951 2,693,416 Butterfield Nov. 2, 1954 2,701,764 Carlson Feb. 8, 1955 2,825,814 Walkup Mar. 4, 1958 2,833,648 Walkup May 6, 1958 2,845,348 Kallman July 29, 1958 

